1. Field of the Invention
The invention relates generally to systems for detecting a thin film layer and more particularly to a system for detecting thin film layers using circularly polarized light.
2. Description of the Prior Art
Objects are often treated with thin chemical films during commercial processing. Sometimes these thin films are removed during the course of the process and it becomes necessary to detect when the thin film is no longer present.
For example, in the semiconductor industry, integrated circuits are produced using thin films of chemicals known as photoresist. Circular silicon wafers are covered with photoresist and are spun so that a thin and even layer of photoresist remains. Light of a certain frequency is passed through a mask and exposes the photoresist. The mask has a circuit pattern which allows only certain portions of the photoresist to be exposed to the light. After the photoresist is exposed to the light, the exposed areas are developed or etched away, leaving the underlying substrate exposed for the next process step. These areas of exposed substrate which contain the circuit pattern may be less than one micron in width and may represent less than two percent of the surface area of the wafer. With these small areas, tolerances become extremely high. If the wafer is left exposed to the light too long, then areas of photoresist in addition to the circuit pattern may be developed and the substrate exposed. It is therefore extremely important to know when the photoresist of the circuit pattern has been fully etched away so that the development process can be shutoff immediately.
The point in time during the process when the photoresist has been completely etched away is referred to in the industry as the "end-point". Various end-point detection apparatus and methods are known in the prior art. These include U.S. Pat. No. 4,647,172 (Batchelder et al.); U.S. Pat. No. 4,618,262 (Maydan et al.); U.S. Pat. No. 4,338,157 (Kanda); U.S. Pat. No. 4,377,436 (Donnelly et al.); U.S. Pat. No. 4,312,732 (Degenkolb et al.); U.S. Pat. No. 4,469,424 (Matsui et al.); U.S. Pat. No. 4,621,037 (Kanda et al.); U.S. Pat. No. 4,389,217 (Baughman et al.); U.S. Pat. No. 4,462,860 (Szmanda); and U.S. Pat. No. 4,634,880 (Lindow, et al.).
The most common type of end-point detector involves aiming a beam of light at a wafer and detecting the intensity of the reflected light. As the thin film layer is etched away, its depth changes, and this causes oscillations in the detected intensity level of specularly reflected light. Once the film layer is removed, the light intensity stabilizes.
These prior art systems have a number of problems. As the circuit patterns used today have become increasingly small, so have the exposed film areas of interest. The light reflected from these areas is proportionally smaller and interfering light from ambient light is proportionally larger. The signal-to-noise ratio becomes smaller. In other words, the oscillations caused by the level of the exposed film layer become smaller and harder to detect.
Another problem with prior art systems is that they require precise positioning of the end-point detector relative to the wafer. The circuit pattern consists of channels etched in the photoresist. Depending upon the orientation of the wafer, the intensity and polarization of the reflected light can vary greatly. In addition, some of the prior art requires that the end-point detector focus on a certain portion of the wafer using a lens. This requires that the end-point detector be positioned a precise distance above the wafer so that the focal point of the lens is located on the wafer.